style: Add some style quirks for legacy ::markers created from list-style-type/list-style-image

Specifically:
For "bullets", i.e. 'list-style-type:disc|circle|square|
disclosure-closed|disclosure-open', we use a built-in font
(-moz-bullet-font, which has glyphs for those symbols + space) to
retain mostly backwards compatible rendering for those.  Authors may
override that with an explicit 'font-family' ::marker style though.
We also use this font for 'list-style-image' in case it would
fallback to one of the above when the image fails to load (so that
we get the same width space).

When the -moz-bullet-font is used we also set 'font-synthesis' to
avoid synthesizing italic/bold for this font. Authors may override
this with an explicit ::marker declaration.

We also set 'letter-spacing' and 'word-spacing' to the initial value
for bullets for web-compat reasons.  Again, authors may override
this with an explicit ::marker declaration. (This breaks backwards-
compat slightly but makes us compatible with Chrome.  We used to
ignore these for list-style-type:<string> too.)

Differential Revision: https://phabricator.services.mozilla.com/D111693

components/style/values/specified/font.rs

@@ -2005,6 +2005,14 @@ impl FontSynthesis {
             style: true,
         }
     }
+    #[inline]
+    /// Get the 'none' value of font-synthesis
+    pub fn none() -> Self {
+        FontSynthesis {
+            weight: false,
+            style: false,
+        }
+    }
 }
 
 impl Parse for FontSynthesis {

components/style/values/specified/list.rs

@@ -67,6 +67,15 @@ impl ListStyleType {
             _ => unreachable!("Unknown counter style keyword value"),
         })))
     }
+
+    /// Is this a bullet? (i.e. `list-style-type: disc|circle|square|disclosure-closed|disclosure-open`)
+    #[inline]
+    pub fn is_bullet(&self) -> bool {
+        match self {
+            ListStyleType::CounterStyle(ref style) => style.is_bullet(),
+            _ => false,
+        }
+    }
 }
 
 #[cfg(feature = "gecko")]